Io, one of the four brightest satellites orbiting Jupiter, has the hottest region in the Solar System on it barring the Sun, although, the average temperature of its parent planet is only -153 degrees Celsius (-243 degrees Farenheit). It is the most volcanically active body and one of the most mysterious bodies in the Solar System.

First of all, Io is hot and volcanic not because of its elliptical orbit but because of Tidal forces, forces caused because each of the particles of Io have to be forced to follow its orbit, in the gravitational field of other objects, in both trajectory and speed when they would have had different trajectories and speeds if left unbounded. These tidal interactions happen mainly between Io, Jupiter, Ganymede and Europa. In the simplest terms, tidal interactions are the same forces which cause tides on Earth. These forces, at any time, cause the region of Io closest to and the region farthest from Jupiter to swell up (expand away from the center of Io) and the region which is in between (neither too close nor too far from Jupiter) to press down (compress towards the center of Io).

Now, the period of rotation and revolution of Io is almost the same. So, just like our Moon, almost the same side of it faces Jupiter. But the gravitational effect of Ganymede and Europa causes it to wobble a bit leading to slight changes in the side facing Jupiter in turn leading to change in that region of Io which is swelled up and which is pressed down. Due to tidal interactions the shape of Io distorts by as much as 100 m whereas the highest ocean tides on Earth only reach about 18 m. This shifting deformation over Io generates a lot of heat by losing energy as what are known as hysteresis losses just like those in rubber, when it is deformed and reformed repeatedly. Hysteresis losses are losses that result from dissipation of energy as heat when bonds in a material deform and then regain the shape. So, the energy transfer isn't from kinetic energy of the rigid body (Io, in this case) to heat energy but from gravitational potential energy of Jupiter-Io-Ganymede-Europa system to electrostatic potential energy of the intermolecular bonds in Io to heat energy.

Io still has so many peculiar features that an Io Volcanic Probe Mission has been proposed to investigate mysteries like how does Io support its high magma temperatures without being almost totally molten and how does it maintain a strong enough lithosphere to support mountains higher than the Mount Everest.

Ritesh Singh,

Indian Institute of Technology Kharagpur.